Question

Why does iodine turn blue-black in the presence of starch?

Answer 1

Apparently, the Iodine gets stuck in the coils of beta amylose molecules. ... Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. In the Iodine test , Iodine dissolved in aqueous solution of Potassium Iodide (KI) reacts with starch producing a bluish colour.

Answer 2

Iodine solution is a solution of elemental iodine in another solution of iodie, usually KI. So present in the solution is the tri-iodide ion. 3 iodine in a straight line with a single negative charge.

Starch can take several shapes, but one of the shapes is like a helix.

When the iodine is added to the starch, the tri-iodide ion is the right shape and size to fit into the helix shape of the starch, so it does so. It forms what is called a starch tri-iodide complex.

This complex has the ability to absorb almost all of the visible light landing on it, so it appears a dark blue-black colour.

This is the basis for quite a sensitive test for both starch and iodine.

If you are testing for starch, even in small amounts, using iodine quickly shows the dark blue-black colour.

If you need to test for the presence of iodine, such as at the end of an iodometric titration, then starch is very sensitive to small amounts, and provides a nice dramatic colour change.